TOKYO — President Biden indicated on Monday that he would use military force to defend Taiwan if it were ever attacked by China, dispensing with the “strategic ambiguity” traditionally favored by American presidents in outlining what the United States would do in such a volatile scenario.

At a news conference with Prime Minister Fumio Kishida of Japan during a visit to Tokyo, Mr. Biden suggested that he would be willing to go further on behalf of Taiwan than he has in helping Ukraine, where he has provided tens of billions of dollars in arms as well as intelligence assistance to help defeat Russian invaders but refused to send American troops.

“You didn’t want to get involved in the Ukraine conflict militarily for obvious reasons,” a reporter said to Mr. Biden. “Are you willing to get involved militarily to defend Taiwan if it comes to that?”

“Yes,” Mr. Biden answered flatly.

“You are?” the reporter followed up.

“That’s the commitment we made,” he said.

The president’s declaration, offered without caveat or clarification, surprised some members of his own administration watching in the room, who did not expect him to offer such unvarnished resolve. The United States historically has warned China against using force against Taiwan while generally remaining vague about how far it would go to aid the island in such a circumstance.

The White House quickly tried to deny that the president meant what he seemed to be saying. “As the president said, our policy has not changed,” the White House said in a statement hurriedly sent to reporters. “He reiterated our One China Policy and our commitment to peace and stability across the Taiwan Strait. He also reiterated our commitment under the Taiwan Relations Act to provide Taiwan with the military means to defend itself.”

But Mr. Biden’s comments went beyond simply reiterating that the United States would provide Taiwan with arms, because the question was posed as a contrast to what he had done with Ukraine. The president made no effort to qualify what he meant when he agreed that he would “get involved militarily.”

In fact, he repeated the notion that his commitment to Taiwan went beyond what he had done for Ukraine. “It’s just not appropriate,” he said of a possible Chinese attack on Taiwan. “It would dislocate the entire region and be another action similar to what happened in Ukraine, and so it’s a burden that is even stronger.”

Mr. Biden had ignored the practiced ambiguity of his predecessors once before in his presidency, stating in similar terms last October that the United States would protect Taiwan. “Yes, we have a commitment to do that,” he said at a town hall-style meeting at the time. That also set off a frantic scramble by the White House to walk back his remark by insisting that he was not changing longstanding policy.

Indeed, the president has made a habit of disregarding the cautions his staff would prefer he take in confronting overseas adversaries. In March, Mr. Biden went further than his administration had gone by calling President Vladimir V. Putin of Russia a war criminal in response to a reporter’s question. Barely a week later, he caused a stir when he ad-libbed a line at the end of a speech in Poland declaring that Mr. Putin “cannot remain in power.”

The Russian invasion of Ukraine has been watched closely in Asia for whatever lessons it would hold for China’s longstanding ambition to reincorporate Taiwan. If Russia had succeeded in conquering Ukraine, once part of its empire, some feared it would offer a dangerous precedent. Yet Russia’s abject failure to take over the entire country and the unified Western response may serve as a red flag to military adventurism.

Mr. Kishida, who spoke in strong terms about China during the news conference, expressed concern about a Ukraine-style conflict over Taiwan. Any “unilateral attempt to change the status quo by force like Russia’s aggression against Ukraine this time should never be tolerated in the Indo-Pacific,” he said.

Nonetheless, he stuck to the traditional policy and maintained before the president’s comments that U.S.-Japan policy on the island was still the same. “Our two countries’ basic position on Taiwan remains unchanged,” he said.

Zolan Kanno-Youngs reported from Tokyo, and Peter Baker from Seoul.